Remote alarm systems are known in the art. These systems consist of a monitoring center and a plurality of remote units which are located in various locations. The remote units are generally connected to the monitoring center via a telephone network.
Telephone based alarm systems for monitoring guarded areas are known in the art, Reference is now made to FIG. 1A which is a schematic illustration of a known alarm system 100. System 100 includes a sensor 101, a remote unit 102 and a central unit 104. The remote unit 102 is connected to the monitoring center 104 via a telephone network 106 and to sensor 101 via a local wired or wireless is communication connection 108.
Remote unit 102 includes an interface 114 for connecting to local devices such as sensors a communication interface 110 for connecting to the telephone network 106 and a processor (CPU) 112 for controlling the unit 102. CPU 112 is connected to interface 114 and to communication interface 110.
The central unit 104 is a monitoring and maintenance center. Monitoring and maintenance center 104 includes a communication interface 116 for connecting to the telephone network 106, a processor (CPU) 118 and a human interface 120. CPU 118 is connected to interface 116 and human interface 120.
The communication over telephone network 106 is bi-directional, hence the remote unit can call the monitoring center in case of an alarm event and the monitoring center can call the remote unit for various procedures, such as routine diagnostic procedures, random check-ups, down load and up load of various parameters, and the like.
Reference is also made now to FIG. 1B which is a schematic illustration of a prior art method, referenced 150, for operating system 100. Method 150 is a conventional diagnostic communication procedure which includes the following steps:
In step 152, the monitoring center 104 dials the telephone number of the remote unit.
In step 154, the monitoring center waits a predetermined number of rings.
In step 156, the monitoring center disconnects the call.
In step 158, the remote unit 102, having detected the rings, dials the telephone number of the monitoring center 104.
In step 160, the monitoring center picks-up the call.
In step 162, the monitoring center 104 and the remote unit 102 exchange data over the network.
According to a similar prior art method, the remote unit, after a predetermined number of rings, answers the call. Then, the monitoring center transmits an encoded identifying signal, the remote unit decodes the signal and compares it with a pre-stored string. It will be appreciated that this poses a disadvantage since an unauthorized party can transmit the identifying signal.
There are several other disadvantages to such methods. When the remote alarm unit is connected to a telephone line to which there is also connected a conventional telephone device, the system has to wait a large predetermined number of rings, usually eight or more, before answering the call, which takes a considerable period of time. The system awaits for a person to pick up the conventional telephone and take the call. If the call has not been answered after a considerable number of rings, then the system will answer the call.
It will be appreciated that, during this waiting period of time, some of the resources of the monitoring center 104 are dedicated to this procedure, and are not available for other purposes. In systems wherein the monitoring center has to control a large number of remote units, a routine diagnostic procedure to several hundreds of remote units may take hours.
This disadvantage is enhanced when the alarm system has to share the telephone line with a conventional telephony device such as an answering machine, a facsimile, a network computer and the like, since each of these telephony devices is programmed to answer an incoming call after a predetermined number of rings.
Another disadvantage is that the remote unit may detect a call from a third party, mistakenly consider it as a call from the monitoring center, and initiate an undesired call to the monitoring center.
Furthermore, when the remote unit is disarmed and there are authorized persons in the protected area, one of these persons may consider the rings provided by the call from the monitoring center 104, as an incoming call and pick up the phone.